1. Field of the Invention
The present invention relates to an image forming apparatus of a transfer type, i.e., an image forming apparatus for outputting an imaged copy by effecting an image forming process including the steps of forming a transferable image on an image bearing member such as a photo-conductive body (photosensitive body), a dielectric body, a magnetic body or an intermediate transfer body by an appropriate image forming means of an electrophotographic type, an electrostatic recording type or a magnetic recording type and transferring the transferable image formed on the image bearing member onto a transfer material such as a recording sheet by means of a transfer means. More particularly, the present invention relates to an image forming apparatus of an electrophotographic system of contact type in which the transfer material is introduced into and conveyed by a transfer nip defined between the image bearing member and a transfer member contacted with the image bearing member and transfer bias is applied to the transfer member to transfer the transferable image formed on the image bearing member onto the transfer material.
2. Related Background Art
In conventional image forming apparatuses of the transfer type, a transfer means for transferring a transferable image (referred to as "toner image" hereinafter) formed and borne on an image bearing member onto a transfer material such as a recording sheet, many systems such as a corona transfer system (noncontact electrostatic transfer type), a roller transfer system (bias roller transfer type, contact electrostatic transfer type) and the like have been used.
(a) Corona Transfer System
In the corona transfer system, as shown in FIG. 6, a corona charger 102 is opposed, in a noncontact relation, to an image bearing member 101 (for example, a rotating electrophotographic photosensitive drum), a transfer material P is introduced into a gap (transfer section) 103 between the image bearing member 101 and the corona charger 102, and a lower or under surface of the transfer material P (surface remote from the image bearing member 101) is exposed to corona shower discharged from the corona charger 102. By corona-charging the transfer material with a charging polarity opposite to that of the toner image t on the image bearing member 101, the toner image t on the image bearing member 101 is electrostatically transferred onto a front or upper surface of the transfer material P. Since the lower surface of the transfer material P is corona-charged by the corona charger 102 in the transfer section 103, the transfer material is closely contacted with the surface of the image bearing member 101 electrostatically. The transfer material P passed through the transfer section 103 is separated from the surface of the image bearing member 101 by a separation means (not shown), and the separated transfer material is conveyed to an image fixing means (not shown). The reference numeral 104 denotes a voltage applying electric power source; and 105 denotes a transfer material guide (pre-transfer guide).
In the corona transfer system, since the image is transferred onto the transfer material P in a condition that the image bearing member 101 is not contacted with the transfer means (corona charger) 102, damage of the image bearing member 101 can be minimized and various kinds of transfer materials can be used. However, since ozone and NOx are generated due to the corona discharge, there must be provided a means for removing the ozone and NOx. Further, since the corona charger 102 is used, the entire apparatus is made complicated and there is a danger of impinging the transfer material P against the corona charger 102.
(b) Roller Transfer System
In the roller transfer system, as shown in FIG. 7, a transfer roller (contact charge member) 106 having conductivity and elasticity is urged against an image bearing member 101 with a predetermined urging force to form a transfer nip (transfer section) N therebetween. The transfer roller 106 comprises a metallic core also acting as an electricity supply electrode, and a semiconductive rubber layer (base layer rubber) formed as a roller-shape around the metallic core, and is rotated in a rotational direction opposite to that of the image bearing member 101 at a peripheral speed substantially the same as that of the image bearing member 101.
The transfer material P is introduced into the transfer nip N and is pinched and conveyed by the transfer nip. While the transfer material P is being pinched and conveyed by the transfer nip N, a predetermined transfer bias is applied from a power source 107 to the metallic core of the transfer roller 106. As a result, a lower surface (remote from the image bearing member 101) of the transfer material P pinched and conveyed by the transfer nip N is contact-charged with a charging polarity opposite to that of the toner image t on the image bearing member 101 by the transfer roller 106 to which the transfer bias was applied, with the result that the toner image t on the image bearing member 101 is transferred onto the upper surface of the transfer material P. The transfer material P passed through the transfer nip N is separated from the surface of the image bearing member 101 by a separation means (not shown), and the separated transfer material is conveyed to an image fixing means (not shown).
In such a roller transfer system, since the voltage applied to the transfer roller (transfer member) 106 can be made smaller than the voltage in transferring devices utilizing the corona charger, generation of ozone and NOx can be suppressed. Further, since the image is transferred onto the transfer material P in the transfer nip N while the transfer material is being pinched between and conveyed by the image bearing member 101 and the transfer roller 106, the conveyance of the transfer material is stabilized. There is less danger of causing transfer deviation due to shock acting on the transfer material when the transfer material enters into and leaves from a transfer material convey means and a fixing means disposed at upstream and downstream sides of the transfer section. For these reasons, the roller transfer system has recently been used widely.
However, since a resistance value of the transfer roller (contact transfer member) 106 is varied with environmental conditions such as temperature and/or humidity and durability, a relation (V-I feature) between voltage applied to the transfer roller and current flowing through the transfer roller is greatly changed in accordance with the environmental conditions and the like. Thus, unless any countermeasure is provided, it is difficult to maintain good transferring ability under all of environmental conditions.
As one of transfer bias controlling systems for solving the above problems, an ATVC (Active Transfer Voltage Control) system has been proposed and used (for example, see U.S. Pat. No. 5,450,180). Such an ATVC system will now be briefly explained with reference to FIG. 8.
First of all, the transfer roller (transfer member) 106 is subjected to "constant current control" with current I1 before the transfer material P reaches the transfer nip (transfer section) N, and the voltage at that time is held. When the transfer material P reaches the transfer nip N, "constant voltage control" is effected with the held voltage.
If the resistance of the transfer roller 106 is small under a high temperature/high humidity condition (H/H, 32.5.degree. C., 85%), a relatively low voltage Va is applied to the transfer roller during the transferring; whereas, if the resistance of the transfer roller 106 is great under a low temperature/low humidity condition (L/L, 15.degree. C., 10%), a relatively high voltage Vc is applied to the transfer roller during the transferring. Further, under a normal temperature/normal humidity condition (N/N, 23.degree. C., 64%), a voltage Vb having an intermediate value between the values Va and Vc is applied to the transfer roller 106 during the transferring. In this way a, substantially desired transfer current can be obtained over the entire environmental conditions.
The improvement in the above ATVC control is disclosed in U.S. Pat. No. 5,179,397. In this control, the hold voltage obtained in the "constant current control" effected before the transfer material P reaches the transfer nip N (between the image bearing member and the transfer roller) is multiplied by a certain coefficient R to provide a control voltage, and, when the transfer material P reaches the transfer nip N, "constant voltage control" is effected with the control voltage. By appropriately selecting the coefficient R, a more proper transfer current can be obtained. That is to say, the bias applied to the transfer roller 106 has a voltage value determined in the so-called ATVC control system by calculation by using a pre-set control equation(s) on the basis of the voltage generated by applying a constant current (under "constant current control") to the transfer roller 106 from the power source 107 when the transfer material P is not present in the transfer nip N.
By performing such control, regarding the bias applied to the transfer roller 106 during the transferring of the toner image onto the transfer material P, even if the resistance of the transfer roller 106 is varied with the temperature and/or humidity, the bias optimum to the changed resistance can be applied to the transfer roller, thereby obtaining a good transferred image.
However, when a transfer member having electrostatic capacity is used as a transfer roller, in the above conventional ATVC control systems and roller transfer systems, since the electrostatic capacity of the transfer roller (transfer member) is not taken in consideration, the constant current control for determining the transfer bias voltage for the transfer member is performed during transient response. Thus, when the so determined constant voltage value (transfer voltage) is used, the current is gradually decreased by the transient phenomenon, so that saturation is reached at a current value smaller than the current value which is actually required. In this case, under a low temperature/low humidity condition in which the resistance of the transfer material becomes great or in a condition that a second surface of the transfer material is copied in a both-face copy mode, an amount of charges applied from the transfer roller to the transfer material becomes insufficient due to poor transfer current, so that it is difficult to correctly transfer the toner image formed on the image bearing member onto the transfer material and to hold the toner image transferred to the transfer material on the transfer material, to thereby cause the poor image.
In this case, in a transfer roller not having an outermost coating layer as is in the prior art, since a thickness of the base rubber layer is generally is great (several millimeters), the electrostatic capacity thereof becomes small to reduce the transient response time. However, for example, in a case where the transfer roller has one or more outer coating layer(s), when the electrostatic capacity thereof is great in comparison with the transfer roller not having any coating layer, the transient response time becomes longer, and, when volume resistance of the coating layer is greater than that of the base rubber layer, the reduction amount of the current becomes particularly great. Thus, the transfer current differs from each other between the case where the transfer bias is determined by the constant current control and the case where the determined voltage is applied to the transfer roller during the image transferring, to thereby cause the insufficient transfer current due to the transient phenomenon. As a result, there is a greater danger of generating a poor image in comparison with the transfer roller having no coating layer.
Explaining in more detail, in the transfer roller having the coating layer, the transfer bias is determined by control as shown in FIG. 9. That is to say, the transfer bias has a voltage value determined in the so-called ATVC control system by calculation by using a pre-set control equation(s) on the basis of the voltage generated by applying a constant current (under "constant current control") to the transfer roller 106 from the power source 107 for a time period T1 when the transfer material is not present in the transfer nip N. By performing such control, regarding the bias applied to the transfer roller 106, even if the resistance of the transfer roller 106 is varied with the temperature and/or humidity, the bias optimum to the changed resistance can be applied to the transfer roller, to thereby realize a good transferred image.
However, in general, when the constant voltage continues to be applied to the transfer roller 106, the transient phenomenon as shown in FIG. 10 occurs. This phenomenon noticeably occurs in the transfer roller having one or more coating layer(s) and can be explained by using an equivalent circuit as shown in FIG. 11. In FIG. 11, R1 denotes a resistance value of the base rubber layer 15b of a transfer roller 15, R2 denotes a resistance value of a coating layer 15c and C denotes electrostatic capacity of the coating layer 15c . In such a circuit, when voltage V is applied, current I flowing through the circuit can be represented as follows: ##EQU1##
Here, in FIG. 10, the current is gradually decreased from a current value Ia (at a time Ta when the constant voltage bias is started) to a current value Ib. According to the above equation (1), when a time at which the current value reaches Ib is Tb, (Tb-Ta) depends upon the resistance and electrostatic capacity of the transfer roller and (Ia-Ib) depends upon the resistance value of the transfer roller. It can be seen that, in the transfer roller having small resistance, the values of (Ia-Ib) and (Tb-Ta) become small, and, in the transfer roller having small electrostatic capacity, the value of (Tb-Ta) becomes small.
For example, when a time period between the time at which the constant voltage bias to the transfer roller is started to the time at which the image transferring is started is short, in some cases, the current is decreased due to the transient phenomenon to cause poor current transfer.
Further, as previously described in connection with FIG. 9, even when the constant current control is effected for the transfer roller to determine the transfer voltage, in some cases, the transient phenomenon occurs. That is to say, as shown in FIG. 5, the voltage is increased from a voltage value Va applied to the transfer roller at the time (Ta) when the constant current control of the transfer roller is started to a voltage value Vb applied to the transfer roller at the time (Tb) when the transient phenomenon is substantially finished. A value of such (Tb-Ta) also depends upon the resistance and electrostatic capacity of the transfer roller. If the voltage applied to the transfer roller is detected on the way of the transient phenomenon, (since lower voltage is detected) it is judged that the resistance of the transfer roller is low in comparison with the actual resistance of the transfer roller, with the result that the poor current transfer occurs in the transferring.
As mentioned above, it is considered that, as the transient phenomenon, there arises a phenomenon in which the current flowing through the transfer roller is gradually decreased from the time when the constant voltage is applied to the transfer roller and a phenomenon in which the voltage applied to the transfer roller is gradually decreased from the time when the constant current is applied to the transfer roller. In this way, in the transfer roller having at least one coating layer, the resistance thereof differs from each other between a case where the control (such as ATVC control) for determining the transfer bias under the transient phenomenon and a case where the determined voltage is applied to the transfer roller. As a result, the transfer current is also differentiated, to thereby cause insufficient transfer current, and, a poor image.
It is considered that the voltage or the current continues to be always applied to the transfer roller for a predetermined time period in consideration of the transient phenomenon before the image transferring and before the ATVC control. However, since the resistance and the electrostatic capacity of the transfer roller is varied with a change in the environmental condition such as temperature and/or humidity, the time when the transient phenomenon is finished is changed in accordance with the environmental condition. Further, when the above-mentioned predetermined time period is selected too long, productivity of image formation is worsened.